The objectives of the current study are to systematically define the alterations of the x-ray attenuation characteristics (as assessed by the computerized transmission tomographic (CTT) scan) of myocardial infarction in the canine heart from the initiation of acute ischemia to its evolution into a fibrous scar. Another objective is to define the interaction of contrast material with myocardial infarctions of varying ages. The light and electron microscopic correlates of the CTT image with and without the influence of contrast material will also be defined. The accuracy of the CTT scan to estimate the volume of infarctions will be compared with the volume defined by histochemical staining and radionuclide markers of myocardial infarctions. The partition of contrast material in the ischemically damaged and normal myocardium and surrounding organs will be defined by fluorescent excitation analysis and correlated with microscopic findings in rabbits and dogs with infarcts of varying age. The mechanism and significance of accumulation of contrast material within ischemically damaged tissue will be explored by defining the tissue locus of the contrast material by autoradiographic electron microscopy, and back scattered scanning electron microscopy in reversibly and irreversibly damaged tissue from fetal mouse, canine, and lapine hearts. While the initial studies are intended to be conducted in ex situ hearts utilizing the cranial CTT scanner, subsequent experiments to evaluate the influence of various interventions on the volume of infarcts as assessed by CTT will be continued using physiologically gated whole body scanner or a rapid scanner.